Abstract
By allowing clay suspensions to evaporate slowly on a flat surface, specimens are obtained in which the particles are mainly oriented with their basal planes parallel to the plane of sedimentation. The basal spacing reflexions obtained from such specimens with an X-ray spectrometer can be identified with more certainty than those obtained from X-ray powder diagrams. This is important as the basal spacings are the best characteristics for differentiating between various clay minerals or groups of such minerals, especially if several of them occur in a mixture and their gran-sizes are small.